1. Field of the Invention
The present invention relates to a voice band extender and more in particular to a voice band extender for extending the frequency band of a band-limited speech signal by adding a signal having its frequency band higher than that of the speech signal.
2. Description of the Background Art
Telephone communication systems conveying speech signals have the frequency band thereof ranging from 0.3 to 3.4 kHz, which is so much narrower than the frequency range of genuine human voices. Therefore, the voice sounds or talker's voice transmitted over telephone systems are somewhat deteriorated to the level of muffled voice, bringing the strangeness of the talker's true voice to a listener.
Japanese patent laid-open publication No. 2003-256000 to Tokuda et al., discloses a telephone terminal unit, which includes a sound signal receiver for receiving a speech signal having its frequency band narrower than that of the true voice sound of a talker over a transmission line of, e.g. a telephone network. The telephone terminal unit also includes a first voice band extending section for shifting the frequency of the narrow band speech signal received by the sound signal receiver to a voiced sound frequency band, a second voice band extending section for shifting the frequency of the narrow band speech signal to an unvoiced sound frequency band, and a third voice band extending section for virtually restoring the low-frequency component of a voice sound lost due to limiting the frequency band of the speech signal. The signals produced by those voice band extending sections are in turn combined with each other to thereby extend the narrow-band of the speech signal to a higher frequency band to produce a speech sound having its bandwidth virtually expanded on the loudspeaker of the telephone handset, thus improving the sound quality of speech signals.
However, a speech signal which the conventional telephone terminal unit receives contains speech signal components of the voice sound of a talker to be extended in the form of non-noise and noise-suppressed signal components, as well as other signal components in the form of noise and extracted-noise components. Thus, the first, second and third voice band extending sections disclosed by Tokuda et al., are adapted to extend an original speech signal with the noise components causing abnormal and harsh sounds still contained therein. The conventional telephone terminal unit combines such extended speech signals just as they are, resulting in deterioration in quality of voice sound produced from the loudspeaker of the telephone handset.